ADEN (Reuters) - The Iran-aligned Houthi movement launched missiles on the Saudi capital Riyadh on Sunday, and reinforced Yemen’s main port city of Hodeidah as an Arab coalition moved closer to the city centre in the largest offensive of the war.

Saudi Arabia’s air defence forces intercepted two ballistic missiles over Riyadh, state media reported, in at least the sixth attack targeting the Saudi capital since December. Houthi-run media said the rockets were aimed at the Saudi defence ministry and other targets.

The Saudi-led military alliance launched its assault on the heavily defended Yemeni Red Sea city of Hodeidah on June 12 to try to weaken the Houthis by cutting off a key supply line for the group, which controls the Yemeni capital Sanaa and most populated areas.

“There is a heavy deployment of armed Houthis in the city and new check points have been set up in neighbourhoods where there are supporters of the Tehama brigades,” said one resident on Sunday, referring to a Yemeni faction from the Red Sea coastal plain that is fighting with coalition forces.

Fierce clashes broke out near Hodeidah University, 3 km (1.9 miles) west of the city centre, on the coastal road linking the airport to the port, added the resident, requesting anonymity.

Coalition forces seized the airport on Wednesday and have been consolidating their hold in the area as U.N. efforts continued to reach a political deal that would avert an assault on the port, a lifeline for millions of Yemenis.

The United Nations fears the escalation in fighting could exacerbate what is already the world’s most urgent humanitarian crisis, with 22 million Yemenis dependent on aid and an estimated 8.4 million believed to be on the verge of starvation.

The World Food Program said the fighting could result in up to 1.1 million people being either displaced or trapped within the city and in need of emergency food assistance.

Medical charity Medecins Sans Frontieres said it received 151 injured people in recent weeks in its trauma centre in the southern city of Aden, many of whom came from Hodeidah, and expects to receive more as fighting moves towards Hodeidah city.

The Western-backed coalition intervened in Yemen in 2015 to restore the internationally recognised government in exile, but since then neither side has made much progress in the war, widely seen as a proxy conflict between Saudi Arabia and Iran.

U.N. ROLE

The Arab states say they must recapture Hodeidah to deprive the Houthis of their main source of income and prevent them from smuggling in Iranian-made missiles. The group and Tehran deny the accusations.

U.N. envoy Martin Griffiths has visited Sanaa and Saudi Arabia to try to negotiate a solution.

The Houthis have indicated they would be willing to hand over management of the port to the United Nations, sources told Reuters. A U.S. official said Washington was urging the Saudis and Emiratis to accept the deal.

Yehya Sharafeddine, vice president of the Yemeni Red Sea Ports administration, said that ships heading for Hodeidah were already subject to U.N. inspections.

“The supervision is already in place even if they’re (U.N.)not present at the port. Anything that enters the port is under the supervision of the United Nations,” he told a press conference at the port, adding that it was operating normally.

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The United Nations has beefed up its inspections of ships bringing humanitarian aid to ensure no military items are being smuggled and to speed delivery of relief supplies, which have been slowed down by additional checks enforced by the coalition.

The coalition has pledged a swift military operation to take over the airport and seaport without entering the city centre, to minimise civilian casualties and maintain the flow of goods.

The International Crisis Group said the battle for Hodeidah was reaching “the point of no return”.